Asphalt has certain physical properties that vary widely with changes in temperature. For this reason, polymers often are added to asphalt to extend the range of its physical properties. Polymers can reduce the tendency of the asphalt to creep and rut in warm weather by enhancing its high temperature viscoelastic properties; and polymers can minimize cracking and stripping in cold weather by improving the asphalt's low temperature viscoelastic properties.
Not all asphalts and polymers form compatible mixtures. Polymer modified asphalt's storage stability is greatly affected by the compatibility of the asphalt with the particular polymer. Incompatible and partially compatible mixtures undergo phase segregation.
Various method have been suggested for making polymer-modified asphaltic compositions that are sufficiently compatible to be acceptably storage stable and that also have the viscosities and softening points in the ranges required for a particular application.
U.S. Pat. No. 4,514,308 discloses a cosulfonated asphalt and a hydrogenated SBR for use as a well drilling mud. As such the composition requires a high level of sulfonation (about 55 wt. % in Example 1) to ensure dispersibility of the polymer modified asphalt composition in water. This characteristic is extremely undesirable in road paving compositions. The present invention specifically excludes hydrogenated versions of SBR.
U.S. Pat. No. 4,591,611 discloses a composition of 100 parts sulfonated polymers (SBR) and from 5 to 125 parts of gilsonite or conventional asphalt although it is stated in '611 that conventional asphalt does not work when substituted for gilsonite. Those compositions are significantly different from the novel compositions of the present invention, which contain only 2-3 parts polymer per 100 parts asphalt.
U.S. Pat. No. 5,348,994 discloses a road paving asphaltic composition containing storage stable base neutralized sulfonated polymer modified asphalt compositions and method of making them using the polymers butyl rubber, styrene-butadiene linear diblock, styrene-butadiene-styrene linear or radical triblock polymers and EPDM. The polymer is present in an amount of less than 7 wt. % and the resulting composition has a viscosity at 135.degree. C. in the range of from about 150 cPs to about 2000 cPs or from about 3000 cPs to about 8000 cPs. The patent does not disclose the use of random styrene-butadiene or styrene-isoprene polymers.